This invention relates to the field of digital transmission and to means for minimizing the effect of errors in the reproduced signal caused by noise on the channel.
It is well known in the field of digitized transmission that, while gains may be made in reducing bandwidth, for example, the effects of noise in the channel may be so deleterious to the quality of the reproduced signal as to make the effort of doubtful value. Therefore, many methods have been tried to reduce or eliminate these effects. One approach has been to send the digital signals in such a manner as to essentially preclude errors from appearing in the final output. For example, in the U.S. Pat. No. 4,055,832, parity bits equal in number to the bits of the signal were sent so that the original signal could be reconstituted exactly, but with an inherent increase in bandwidth or in the required data rate.
As shown in the book "Digital Audio Technology" by Nakajima et al, 1985 TAB Books, Inc. the effects of error in the signal as received can be reduced in a number of ways. Typically, when a block of data is detected as containing error, it is deleted and a substitution is made. The substituted block may be a zero value signal, a signal having some nominal value less than the expected signal, or a repetition of the last previous good block of values. More sophisticated techniques have used the nearest valid values on each side of the erroneous block to create a parabolic curve of values which matches the valid information at the ends. In the case of a digitized audio signal, this latter type of approach reduces the clicks which might be noticed in the simpler forms of substitution, but requires substantial amounts of memory and calculation.
It is to be noted that, in the context of this description, "detection of error" typically means comparing an "update" value which is presumed to be correct with the corresponding received value.